Leviticus 16:1-2

The Day of Atonement 1The LORD spoke to Moses after the death of the two sons of Aaron who died when they approached the LORD .

2The LORD said to Moses: “Tell your brother Aaron that he is not to come whenever he chooses into the Most Holy Place behind the curtain in front of the atonement cover on the ark, or else he will die. For I will appear in the cloud over the atonement cover.